X-ray tube



T. H. FORDE r-:T AL 1,876,049

X-RAY TUBE sept. 6, 1.932.;

Filed July 8, 1929 Hammmmm. u .l

nmrmipuunuunnannummi )in INVENTORS rHo/lms FoRoE BY NEwro/v vv. NELL/ms THE/R ATTORNEY Patented Sept. 6, 1932 l THOMAS n. FORDE AND NEWTON W.V1VLEL'LARS, on `SAN FRANCISCO, oAIilron'nrA, AssIGNORe To FORMELL CORPORATION, LTD., 0F SAN FRANCISCO, OALIFOiiNIA, A

CORPORATION OF CALIFORNIA.

X-RAY 'runtA Y Appncationmed Julye, 1929. serial 376,694. ,A

The invention relates to the Vtherrnionic emission type of X-ray tubes and more particularly toimprovements in tubes of the class forming the subject of our application Serial Number 299,650 filed August 15, 1928.

An object of the invention is to providel an X-ray. tube adapted ormedical and other lpurposes operable at comparatively low voltages. 1- Y i Another object of the invention is to provide an X-raytube whose operation is independent of small variations inthe vacuum.

A further object of the invention is to provide an X-ray tube utilizing thermionic emission without using a separate source for heating the emitting surfaces.

A still further object of the invention is to ,provide an X-raytube of thel type described in whichv the main current through the tube 1 isutilized for heating thegthermion-emitting members. Y v 4 Y l Another object of, the invention isto provide anX-raytube of the classv described in `which theheat in the thermion-ernitting sur- .faces is generated by induced currents or elecing a therrnallyl and` electricallyinsulated pole placed in the path .of the electrons in close proximity to thecathode, and arranged to .serve as a thermionicA emission member.'Y ,Another object of the invention isl to provide an K-rayA tube of theclass 4described hav- .ing a cup-shaped cathode, a thermionic emission member placed in the path of the lelectrons issued from the cathode, and a member suo orted at an inner oint of the cathode i l)` cup and electrically connected to the latter ,to provide a part of the electric path, the

,adjacent parts of said lastmember and of the therlnionicV emission member` being of the Ysame `general outline so. as to'inrsure the most .efficient electronic pathway between theV two members.

- Anotherobject-ofthe inventionis provide 4an X-ray tube of theclass described having a, cup-shaped cathode and a hehcally A,tors entering the tube.

shaped member supported at the central inner point of the cup, said member carrying a part of themain tube current-being consequently heated by it, and thusgiving risel'to therinionic emission,

, Another obeCt of the invention` is ,to provide an X-ray tube possessing. current rectiltying` properties, i the rectication being, effected by thermionic members Vwhose heat energy is derived from the main tube current'.

Another objectief the invention is to prol-` vide an kX-ray tubeiof the classV described having an auxiliary pole felectrically connected to the target and placed in the V,elec-y tronic path in iront of the targetfsofas to kraise the voltage gradient near the thermion-ic i' members.

Another object of theinvent-ionis toproi vide an internal coIiStruction of a thermi'onic type oX-ray tube which will make practical the use and operation of a tube of an unusu- X-ray generatingportion of the tube may be 'readily insertedinto cavities or orifices of the human body.

Afurther object of the invention Ais to pro-v vide an X-ray vtube of the;thermionic emission type having only two electrical conduc- A still further andanimportant obiectof the invention is to provide an .X-'raytubeot ally small cross-sectional area, whereby the 4such external shape asto be adapted for. intrabodily X-ray therapy, -iuorosoopic examinations, and radiographing parts of thehur'nan body, whereby only relatively small thickness l of tissues vneed be penetrated by X-.rays-thus fenablingthe operator to usesoft X-rays with th .COhSequeht lessenng vof danger ermelogenic burning,` i l The invention possesses otherV objects ,and r1 ieatu'resY of advantage, someof which, with the foregoing, will beset forth in thevtollo'wing description of :the preferred form of .the`

invention which is illustrated in the drawing `accomparlying and orining'part of YtheQspe-f. .cication. It'r is` to be `understood, however, f

that variations in thershowing made bythe said drawing and descriptioninaybe adopted within the scope jot the invention asgfsfet VAtor-th; theclaiins Referring to said drawing:

Fig. 1 is a longitudinal sectional view showing an vembodiment of the X-ray tube of our invention.

Fig.V 2 is a transverse sectional view taken on the line 2--2 in Fig. 1.

`Fig. '3 is an enlarged plan View of `a portion Io'ffthe structure shown in Fig. 1.

` Y if'tors 11 and 12'which, as hereshown, Yalso is va front view of the 1portion shown in F ig.'3. Y

VAs illustrated in the drawing, the X-ray tube 5 of our invention comprises a hollow body 6 preferably yof constant circular section and defining an evacuated chamber. The tube contains two electrodes-a cathode v7 yand an anti-cathodeS-#whi'ch latter lprovides a t-arget 9. Electrical connection for the target and thecathode with a suitable source of power, such as the secondary of atransformer, is established respectively through conducparallelism of the two conductors and to Y A4.o

'render the structure rigid a plurality of sup- `ports made of suitablenon-conducting material, such as glass, isV interposedbetween the conductors, theupper and lower ends of each support beingv preferably fused to insulators 20 and 10' respectively.` In the present embodiment of our invention, as clearly shown in Figure 1, the X-ray emittingmem- Yber-the target-is placed adjacent the operative end 13 of the tube whilel the cathode is located furtherl along the axis of the tube. This is extremely important since withy such arrangement X-rays may be most conveniently generatedin parts of cavities of the human body diflicult of access.

' Y Y As is conventional,the target consists of a block 14 of tungsten or other suitable material, the operative surface 15 of whichis arranged symmetrically with respect to the.

axis' of the tube and vdefines with the latter an angle of about 45 degrees. The support for the. block 14 is conveniently provided throughanrangular extensionV 16 ofthe con- "duc'tor 11, the adjacent'surfaces of the block Yandthe extension being in closemetallic contact'with each other.` The cathode, as here shown, is substantially in the form, of a lhollow spherical shellor cup 17 whose axis "coincides with the axis of the tube, and whose concave surfaceis arranged to focus the lines o fforce issuingfrom the cuprat the'central point of the target. Extending towards the transformer end of the tube and formingzan Leido-1e integral part of the cathode is a hollow mem*- ber 18 which is arranged to lit over the end 19 of the conductor 12, said end being in the form of a frustrum of ya cone with the narrower end of the frustrum extending through a perforation 21 in the central portion of the cathode cup and thus serving'as the bottom 22 of the cup. Itis clear from the above that a double function of supporting the cathode and servingas an electrical lead thereto is conveniently 'exercised by `a 'single conductor 12.

Preferably supportedv at the inner central point of the cathode cup 17' and extending outwardly therefrom lalong the axis of the tube is a member 23 here shown in the form of a helical coil of tungsten wire, and having l its axis common with that of the tube.

Facing the free end of thecoil 23 and in fairly close proximity thereto is a neutral pole 24 which is placed at the axis of the tube. In the present 'embodiment the pole 24 comprises a closed tungsten wire loop 25 hav- .ing its axis at right angles to the axis of the tube and arranged vto support a relatively A small target'26. The latter is conveniently made in the form-of a thin circulardisc of a f diameter equal to that of the coil 23 and having itsaxis identical with the axis of the coil. The poley is suspended by a wire 27 `whichis preferably fused into the free end ofa nonconducting arm 28 of the insulator 20.

Positioned furtheralong the axis of the tube, that is, between the neutral pole 24 'and the target 9, is a relatively small auxiliary pole 29 which isy attached tota wire 31 secured'to an uninsulatedv part 32 of the conductor 11. The pole 29 is thus, as here shown,

in electrical contact with the target and will at all times be at the same potential asthe latter. Y

The tube of my invention, like the Coolidge type, utilizes thermionic emission as the source of electrons, thus being practically independent of small variations of the vacuum,Y

and, as in the latter type, by placingthe emission members near the cathode and properly directing lthe electric field,` self rectification is provided for. An importantV differ- Y ence-between the two types of tubes, however, lies in the fact that in the tube of our inventionno separate source is necessary for heating the thermionic members. All the required energy for-operating the tube is, as here shown, arranged to be supplied directly from the two treminalsV of the secondary of a transformer. .This arrangement fen- Vables us to have only two current carrying membersY entering the Vtube instead oftheA three or more that 'are usedin 'any other tube utilizing thermionic emission. Those skilled in the art can appreciate the greater facili-` ties of insulation made possible bythe reduction-in the number of electrically independentconductors carriedvinsidethe tube. OnV

thevother hand-itis quite obvious that` the -lesser numberof conductors permits the use of a tube of smaller diameter, since the latter visprimarily controlled by the separation of conductors at differing potentials. The iinportance ofthe lsize of X-'ray tubes willbe taken up later. l

It being clear from the above that .alternating voltage isA impressed on the two conduc- Aof thetube. e

'After a short interval, however, the coil 23 tors l1 and 412 connected to the target and cathoderespectively, our next4 step kwill be to explain-how our-invention has madefit pos- 'sible to utilize the power carried'by only two 'conductors for both the heating of the thermionic members and for giving the targetbombarding electrons the necessary velocity ffor production of X-rays.

"leshallfdiscussat `first the conditions existing during thebrief but frequent intervals l'when the cathode is negative and the target positive, `and we shall term this polarity the normal polarity. When thev tube is cold it shouldbe started ata voltage higher thanits 'operative value. However, even the starting voltage need no t be very high Vas the electronic path between the cathode and the target is greatly shortened by the yintroduction of the lcoil 23a-nd vthe auxiliary pole 29. Electrons supplied from the conductor 12 pass through the coil 23 heating itthereby and are issued lfromA the free end of the coil along the lines of forcefocused at the center of the target as determined by the conical electric field ldefined by the cathode cup 17, this focusing being Vthe most important function of the cup. Proceeding with enormous velocities towards the target, some ofithe 'electrons are abruptly fstopped bylthe neutral pole 24 and give up their kinetic energy in the form of heat-raising the temperature of the pole. The temperatureofthe latter is further raised by the leddy currents circulating inthe loop 25 vdue to the fluctuations of the magnetic field "threading the loop, said fluctuations being clearly due to changes in the magnitude of the main tube current( The portion of the 'electronicstream which `was not stopped by the'neutral pole passes practically unobstructed by the very small auxiliary pole .29,

towardsthe anode. As thelresult of the irn- Lpact upon the target, fairly hard rays will :be emitted by the target corresponding to the high starting voltage, itfbeing known that the hardness of rays depends primarily upon the potenti al diiference between the electrodes Vwith velocities of electrons the member 12,*the thermally insulated pole 24 will remain hot. The .voltage across the tube vmay now be considerably reduced, `no further necessity existing for high potentials to tear electrons from the cold metal'surjfaces. The operator is thus free to choose the operative voltage to correspond to the hardnessof l X-rays required.

Dueto the vproximity of the neutral pole '24 and the freeend of the coil23, the electrons leaving either member 'and proceeding towards the target will have to -fall substan-` tially through the total `voltage across vthe tube. Thus all the electrons comprising the electronic stream will reach the target with practically the same velocity, the slight difference in initial velocities of electrons from the poleand thecoil respectively due to a 'difference in temperatures of themembers being entirely insignificant, since it is anestablished fact 'that the initial; velocites Vof thermions are negligibly small incomparison fallen through thousands of volts.

Besides contributing toy the homogeneity,"`

of the electronic stream, the location ofthe pole and the coil at nearly the same point of the electricfield represents an important fac-- tor in the selfrectifying vfunction of the tube. Thus when thewave of the alternating voltage impressed on the tube causes the target to become temporarily negative and the cathode positive, the conditions forthe electronic passage through the tube are radically different from those prevailing with the normal polarity ofthe electrodes. `The voltage reduced to its' operative value is not suilicient to produce an electronic current of any appreciable magnitude from the comparatively cold target serving for the insta-nt asthe negative terminal. e It is amply high,

however, to drive free thermionsto `a posi- Y tivetterminal which in this case is'the cathode,

or even tear off certain number lof electrons from a hot electrode such as theneutral `pole 24. The neutral pole, however, cannotfunction as a source of electronssince it is electrically insulated, 1t being remembered that with the normal polarity the' pole 24 was '115 warm coil 23.y With the reversed'polarity Y 'supplied with the electrons by the adjacent the pole 24 can lose but a limited number `of electrons soon becoming highly positive and thus tending to attract rather'than repel the `negative electrons. It follows that: even with an alternating voltage appliedat the'terminals ofthe tube, thecurrent through the latter -`will vbe substantially f unidirectional, namely from the target to the cathode,or, stated in other words, electrons willalways ltravelV towards the target.` V7 hat few electrons reach the cathodeduring-the reversed polarity, will `not'be lable to generateV Xfrays for several reasons.F irst,the, numberof electrons will be small;` second, their velocity will be very low since electrons in their travel from the neutral pole to the cathode fall throughbut small difference of potential; and third, due to the substantially conically .shaped eld the electronic impacts will be distributed overthe whole surface ofthe cup. It will of course be clear that while here we described the operation of the tube with alternating voltage impressedacross the tube, partly rectified alternating voltage, or even direct current voltage, may be employed with at least equaladvantage.v y

It will now-beclear'that the. tube of our invention maybe made of extremely small size without. sacrificing the eiiiciency ofrits operation. The small sizeof X-ray tubes is an extremelyV important factor; in medical work. F or instance, a smallktube may be inserted into cavities and orifices of the human body, thus making X-ray treat1nent,.iluoro scopic examinations, and photographing, highly localized and precise. Moreover, as

a direct consequence of the-possibility of inserting a small tube intocavities ofthe .hu-

lman body,V there arises the possibility of using the tube at a lower voltage than has heretofore been employed.- Thus during a ffluoroscopic examination of teeth a small tube can be placed in the mouthdirectly vagainst the gums, Vwhilel the screen may be arranged to be in contact with the cheek. Consequently X-rays havetoY penetrate but a small thickness of tissue and may accordingly Vbe of low penetrability, that is, produced by a low voltage across the tube. It

,follows that using the tube of our invention vthe danger of pathogenic burning is greatly minimized, while the danger of electric shock to the operator or the patient is entirely obyviated. d a

' Of at least equal-importance with the reduction in size of the tube as aforesaid, is the feature of arranging the X-ray emitting surface adjacent an extremity of the tubewhich I extremity is adapted for insertion in relatively small cavities of the.l human body or the like. The importance of this last feature Y cannot be over-estimated since the possibility vofbringing the X-ray emitting surface into a close proximity to the tissue to be penetrated by the rays obviates the necessity for We claim.:

using hard X-rays and high voltages.

l.,Y In van X-ray tube, envelope, a

cathode, an anti-cathode, a coil lelectrically connected to the cathode and extending in the general direction vof said anti-cathode, land a thermionic emission memberV comprising a relatively small target interposed between said coil and the anti-cathode to serve as a source of anti-cathode-bombarding elec- Y trons, thefreeend of said coil being adjacent said target and having approximately the 65tsameoutline1as'the target. v Y

tween4 the target and the cathode,r therebyz raising potential gradient throughout the tube without the necessityy of changing the length of the electronic path from the cathode tothe target, and thermionic emission memi bers but one of whichis electrically connected .withthe cathode and both ofwhich are disposed between said pole and said cathode.-

,4. In an .X-ray tube, an envelopeV a cathode,an anti-cathode, a coil ,electrically connected to the cathode and extendingin the general direction of said anti-cathode, a thermionic emission member interposed between said cathode and anti-cathodeand electrically and thermally insulated from all operative parts of the tube, and'avconductor connected tothe cathode, a tubular insulator mountedlto Yelectrically protect said conductor and having a non-'conducting arm, land a wire supporting said "emission member and having an end fused into the freeeend of* said arm.

5. In an X-ray.tube, a cup shaped cathode,

a metallic tubularextension onA said cathode, a conductor engaged inside said, extension Vand supporting said cathode, and a metallic vcoil electrically connected withu and fastened directly to the portion ofy said conductorA engaged in.. said extension-. Y' Y 6. In an X-ray tube, an envelope, a cathode, an anti-cathode, Va coil electrically connected tothe cathode land extending in the general direction of said anti-cathode, a pair of conductors for connecting thepcathode and the anti-cathode respectively to a source of electric power, insulators to electricallyy protect said conductors, and a plurality Hof nonconducting supports interposed between said conductors to maintain the latter in a `predetermined relation toV each `other `and to the envelope. v y v In testimonyI whereof, vwe vhave hereunto set our hands atl Oakland, California, this Y 5th day of July, 1929.v Y

' THOMAS II. FORDE. I

NEWTONV'VW.

2. In an X-raytube a cathode, an envelope, y y

iso 

